To keep a wearer safe and comfortable, footwear is called upon to perform a variety of functions. For example, the sole structure of footwear should provide adequate support and impact force attenuation properties to prevent injury and reduce fatigue, while at the same time provide adequate flexibility so that the sole structure articulates, flexes, stretches, or otherwise moves to allow an individual to fully utilize the natural motion of the foot.
Despite the differences between various footwear styles, sole structures for conventional footwear generally include multiple layers that are referred to as an insole, a midsole, and an outsole. The insole is a thin, comfort-enhancing member located adjacent to the foot. The outsole forms the ground-contacting element of footwear and is usually fashioned from a durable, wear resistant material that may include texturing or other features to improve traction.
The midsole forms the middle layer of the sole and serves a variety of purposes that include controlling potentially harmful foot motions, such as over pronation; shielding the foot from excessive ground reaction forces; and beneficially utilizing such ground reaction forces for more efficient toe-off. Conventional midsoles may include a foam material to attenuate impact forces and absorb energy when the footwear contacts the ground during athletic activities. Other midsoles may utilize fluid-filled bladders (e.g., filled with air or other gasses) to attenuate impact forces and absorb energy.
Although foam materials in the midsole succeed in attenuating impact forces for the foot, foam materials that are relatively soft may also impart instability that increases in proportion to midsole thickness. For example, the use of very soft materials in the midsole of running shoes, while providing protection against vertical impact forces, can encourage instability of the ankle, thereby contributing to the tendency for over-pronation. This instability has been cited as a contributor to “runner's knee” and other athletic injuries. For this reason, footwear design often involves a balance or tradeoff between impact force attenuation and stability.
Stabilization is also a factor in sports like basketball, volleyball, football, and soccer. In addition to running, an athlete may be required to perform a variety of motions including transverse movement, quickly executed direction changes, stops, and starts; movement in a backward direction; and jumping (vertically or with both a vertical and horizontal component). While making such movements, footwear instability may lead to excessive inversion or eversion of the ankle joint, potentially causing an ankle sprain.
High-action sports, such as soccer, basketball, football, rugby, ultimate, etc., impose special demands upon players and their footwear. Accordingly, it would be desirable to provide footwear that achieves better dynamic control of the wearer's movements, while at the same time providing impact-attenuating features that protect the wearer from excessive impact loads.